Technical Field
The present invention relates to an impurity introducing method, an impurity introducing apparatus, and a method of manufacturing a semiconductor element.
Background Art
Semiconductor elements using silicon carbide (SiC), and 4H silicon carbide (4H—SiC) in particular, are showing promise as power semiconductors. A 4H—SiC semiconductor element is normally manufactured by doping a semiconductor substrate on which is formed a 4H—SiC crystal layer expitaxially grown at a desired density with an impurity element such as phosphorus (P), aluminum (Al), or the like. More specifically, for example, the semiconductor substrate is implanted with accelerated impurity element ions, and the semiconductor substrate is then subjected to a heating process (annealing) to restore the crystal structure of the ion-implanted semiconductor substrate and activate the impurity element.
Here, in the case where a (0001) surface (a (000-1) surface) of 4H—SiC is implanted with a high dose of ions (approximately 1015/cm2, for example), it is necessary to carry out a heating process that heats the semiconductor substrate to approximately 300-800 degrees in advance. This is because recrystallization of the 4H—SiC and the activation of the impurity element will not occur effectively without the advance heating process.
SiC annealing is carried out at a higher temperature than for silicon (Si), at approximately 1600-1800 degrees, and such annealing is known to cause Si to fall from the surface of the semiconductor element, induce roughness in the surface of the semiconductor element due to migration, and so on. Accordingly, there is a method that carries out annealing after first forming a protective film of aluminum nitride (AlN), carbon (C), or the like on the surface of the semiconductor element, but such a method is problematic in that forming and removing the protective layer increases the number of processes and furthermore increases processing costs. Surrounding areas being soiled by aluminum (Al), carbon (C), or the like is also a problem of concern.
The laser doping technique disclosed in Non-patent Document 1 can be considered as a method for solving the aforementioned problems. In Non-patent Document 1, a 4H—SiC semiconductor substrate is immersed in an aqueous solution containing an impurity element and a border region between a surface of the semiconductor substrate and the solution is irradiated with laser beam pulses, locally heating the semiconductor substrate and doping the semiconductor substrate with the impurity element present in the solution. A beam having a wavelength in the ultraviolet range, which has a high absorption coefficient with respect to SiC, is used as this laser beam. According to Non-patent Document 1, doping can be carried out with the implantation of the impurity element and the activation of the semiconductor substrate occurring simultaneously, even in an environment equivalent to room temperature, and thus subjecting the semiconductor substrate to the advance heating process and post-impurity element implantation annealing as described above is considered unnecessary.
However, the technique of Non-patent Document 1 has a problem in that the impurity element can only penetrate the semiconductor substrate to a maximum depth of approximately 40 nm from the surface thereof. This is the same regardless of whether a single irradiation target region is irradiated with a single shot of the laser beam pulse or one hundred shots of the laser beam pulse.